Basic to the operation of modern machinery is the transmission of mechanical energy from source locations to points of utilization by means of rotating shafts. In a typical machine the energy is first imparted to a rotating shaft after conversion from chemical, thermal, electrical or kinetic sources within some prime mover such as an engine, turbine or motor. Machines often contain systems of shafts whose rotational motions are interconnected by couplings, belts, gears, or related devices in order to better match the prime mover to the load or to distribute the energy to a multiplicity of loads. Clutches between shafts allow for purposeful decoupling of their rotational motions. The mechanical energy imparted to the output shaft of the prime mover eventually is used to perform useful work in forms and at locations that characterize the function of the specific machine, e.g., propulsion of a vehicle, compression of a fluid, forming or machining of a manufactured part, electrical generation, etc. The ubiquity of utilization of rotating shafts to transmit and distribute mechanical energy is readily illustrated even with the very abbreviated listing in TABLE 1.
TABLE 1 ______________________________________ Machines transmitting power with rotating shafts. INDUSTRY TYPICAL MACHINE ______________________________________ Agriculture Tractor, Combine, Harvester Construction Concrete mixer, Crane, Excavator Food Mixing, Bottling, Canning Lumber/Paper Sawing, Planing, Pulping Mining/Oil Boring, Loading, Pumping Manufacturing Machine tools, Conveying Metals, Plastics Rolling, Slitting, Extruding Recreation Ski lift, Amusement park rides Textile Weaving, Knitting, Sewing Transportation Land, Sea, Air vehicles ______________________________________
The rate at which work is performed is termed Power. Power is also defined as "the time rate of transferring or transforming energy". When the mechanical energy performing the work is transmitted by a rotating shaft, power describes the rate of energy flow along the shaft. Transmitted power is thus clearly a measure of the functionality of a rotating shaft. From this perspective it is clear why "output power" is the primary quantitative factor used to rate both mechanical and electrical prime movers. It is also understandable why so many shaft driven machines, such as pumps and compressors, spindles on lathes, mills and grinders and other machine tools, and even some appliances such as vacuum cleaners and garbage disposal units, are often sized and compared by their power capacities. On-line measurement of the power actually being transmitted along key shafts in a machine can, by quantifying the machine's performance, enable its more precise control and adjustment and also help to ensure its safe and efficient operation. Noted departures from normally generated or utilized power can even provide an early indication of a developing fault.
The importance of on-line power measurement on rotating shafts in working machinery has long been recognized with the resulting development of more or less standardized measurement methods and apparatus. Since the power transmitted through a cross section of any shaft is the product of its instantaneous angular velocity and the torque transferred across the section, the measurement of this power generally reduces to the separate measurement of these two, more basic, quantities. Whatever technologies and specific types of rotational speed and torque measuring devices are actually employed, the determination of power still requires the multiplication of these separately measured quantities. Conventional power measuring instruments therefore include in the overall apparatus, besides means for measuring both speed and torque, some computational circuitry for on-line multiplication of these two, separate signals.